Apparatus and process for production of pulp



M y 1932- J. w. STEVENS ET AL 1,860,755

APPARATUS AND PROCESS FOR PRODUCTION OF PULP Filed Jan. 27, 1928 51 E g9 E olccurr zlula or M 19 uor fnvenifi rs J CU 675606716 61/ 3f ,Kehcty.

Patented May 31, 1932 UNITED STATES PATENT OFFICE- Jnamr w. STEVENS AND WILLIAM H. KENETY, or cnooun'r, MINNESOTA, ASSIGNORS 'ro cnmnrun rnocsss me, or wArnn'rowN, NEW YORK, A CORPORATION or NEW YORK APPARATUS AND PROCESS FOR PRODUCTION OI PULP Application filed January 27, 1828. Serial No. 249,845.

tions in the sulphite process, to the relief of the digester, to the utilization of the heat in the materials relieved, to the recovery of the 7 materials, and to the combination of such utilization and recovery with said prelimi- 1 nary operations.

In the cooking of wood by the sulphite I process, using any of the suitable bases, of which calcium, magnesium and sodium are commonl known, it is necessary to use an acid sulp ite and an excess of free sulphurous acid dissolved in the cooking liquor. The excess acid usually varies in amount. It is not constant, because it is held very loosely in the liquor, and increased heat and decreased pressure both cause it to be discharged from solution in the li uor. It is therefore customary to charge a dlgester with a cooking liquor in which the free acid content is very high. The tendency of. the subsequent heating to drive it off is somewhat balanced by the tendency of the increasing pressure to prevent its escape. In the present practice of the sulhite process where steam is injected into the iquor there is an escape of much free acid from the process, and this is accompanied by discharge of a quantity of the cooking liquor. Vents or relief lines suitably place about the digester have valved controls to regulate the escape of such material and to control the pressure.

It is quite desirable to have all the chips in the digester covered with the cooking liquor,

and accordingly the digester is quite full.

The injection of the heating steam causes condensatlon, and in order to accommodate the increased volume of water resulting. from such condensation a vportion of the liquor is released. It is customary to bleed liquor from the digester at a rate which will compensate for thewat'er being added. This liquor and theacid which is also set free are greatest 1n amount at the beginning ,of the heating operation and in the early part of the cook, but nevertheless. a substantial amount of gas and vapor escape during all the cook; The material first removed by rellef 1s of greater purity than that which is later relieved, for after the cooking has proceeded there are present in the liquor various impurities removed from the chips, such as pitch, lignins, dissolved resins, oils, organic acids, etc.

In the preliminary operations of the sulphite process the liquor is usually introduced so that heating begins at a temperature of about 70 (1., and cooking action does not begin until three or four hours after the heating commences. This heating period in-' volves the introduction of, much water as steam, and requires the displacement of an equivalent amount of liquor, with the disad-' vantage that the original liquor is diluted.

Much of the cooking value in the liquor both as dissolved solids and as free acid, is thus lost through the relief system before any cooking ta es place, and the loss continues as cooking occurs. Much heat is car- {iieg away from the digester during the re- Various processes have been proposed for conserving the chemical value and the heat of the relieved materials, such, for example, as the introduction of them into liquor being prepared for a subsequent cooking operation. Absorption of the free acid gas at atmospheric and at greater (pressures has been proposed, but there are sadvantages to these operations. At atmospheric pressure the temperature which can be attained is limited. At higher pressures the back pressure increases and a constant and uniform relief cannot be maintained. Furthermore, the introduction of the relief materials into the new liquors contaminates them with the impurities which they carry, and there is thus an accumulation of detrimental materials which gives low grade pulp in subserauent cooking operations with such liquor. he present invention aims to use the relief materials for preheating and for enriching raw liquors, so'that a substantial part of the heat is recovered, all the free acid utilized, a substantial part of the high quality cooking chemicals of the relief conserved, and the low grade contaminated relieved chemicals kept away from the raw liquor and otherwise disposed of.

The practice of the present invention pro duces a high grade cooking liquor which is rich in sulphurous acid,'which is heated to heat and the chemical eificiency of the process. In addition to all these advantages the time of operation is reduced, first, by cutting ofi a considerable period of preliminary heating, and second, by cuttin down the cooking time as a result of the stronger liquor retained during the cooking period. An improved-and more uniform pulp is obtained by reason of the more rapid impregnation of the chips by the hotter and stronger liquor prior to the cookin stage, and the more ra id and uniform coo ing of the more thorouglily impregnated chips.

Various other advantages of the resent invention will be apparent from the following description of the process and the apparatus which is hereinafter given as an example of the invention as it is applied specifically to the sulphite process using a calcium base, and as shown in the accompanying drawing.

' In the drawing there is shown a digester,

a separator arid an accumulator, with various connections for suitable operation of the process.

In order to explain the invention and to enable others to comprehend the scope of the appended claims adetailed reference to the apparatus and to its operation is given.

A digester 10, as commonly used for the sulphite process, being roughly fifty feet in height and about fifteen feet in diameter, is arranged to be charged with chips from a supply 11 through a removable top 12. The bottom of the digester is fitted with connections for a blow line 13, a steam injection line 14, and a liquor supply line 15. Such connections may of course be otherwise made without in any way departing from thepresent invention. Suitable valves are provided in these lines, for example, 16 in the blow line, 17 in the steam line, and 18 in the liquor supply line. erally employed in the art, are connected to the digester, in the present instance line 19 with valve 20 being located near the top for the relief of gas and vapor, and line 21 with valve 22 being located lower on the side of the digester for relief of liquor.

The two relief lines unite beyond their valves into line 23 leading into a separator for the separation of liquid and gas and/or vapor. In the present instance a horizontal cylindrical tank about eight feet long by two feet in diameter is designated 24, with line 23 entering one end at the center. The bottom of the tank is connected for drainage to a line 25 with valve 26. This drainage line may lead to a waste receiver or to the usual liquor cooler used for sulphiting apparatus, or to any other means for disposal of the separator liquor. Liquid, or gas and vapor which may collect above a liquid level in the separator, is conducted through line 27 to suitable means which will utilize the chemical and heat value in accordance with the present invention. l

The present invention provides what is hereinafter termed an accumulator. This has a multiplicity of functions, all of which will be clear fromthe following description Various relief lines, as gen-' of its construction, its connections, and its eration, as a preheating pressure chamber for raw liquor. and also as a pressure blow case for filling the cooking digester. In the present instance it may serve as a gravity feed supply for digester 10.

The accumulator is preferably provided as a pressure tank 28 of steel, and it is lined as indicated at 29 to render it acid resistant. It has a valved vent 30 at the top, a liquid level gauge 31, and a bottom drain or discharge line 32 with control valve33. In size it is preferably sufiiciently large to hold a full charge of liquor for digester 10, but it may be smaller and other raw liquor to the amount required may be drawn from the ordinary sources of supply. An old digester is suitable as an accumulator, but when special installations are employed it is desirable to' the accumulator. "This line 32 is connected to a pump 34through valve 35, and also connected to the feed line 15 for digester 10.

The pump receives raw liquor from a suitable supply through line 36.

The accumulator is so connected to the separator discharge line 27 that the relief material in the line may be discharged into the accumulator or may be passed through the accumulator without admixture as through coil 37. The coil is made of acid resistant materialsuch as copper orlead. The preheating coil may be cut off by the valve 38 leading to one branch of a Y-connection 39 mounted on the end of line 27 the other branch being connected into the accumulator by line 40 with valve 41 for discharge at the open end 42. Line 27 may thus be connected alternately or concurrently for discharge into the accumulator. and through the coil. Coil' 37 emerges from the accumulator as pine 43. A valve 44 may be placed in the line 43 in order to insure that the gas and vapor in the coil through the'accumulator has a sustained pressure to maintain its temperature for proper preheating of the raw liquor in the accumulator. A sufficient length of line 43 may make such valve unnecessary. Usually suflicient length is to be found when the line leads to the cooler which is used in connection with the usual sulphiting towers, to which cooler it is indicated as bemg connected for saving the sulphurous acid. In order to cut out the coil when desired, a by-pass line 45 with valve 46 is employed. Valves 44 and 38 are thus useful in cutting out the coil at both ends, yet permitting discharge into the accumulator through pipe 40. It is to be understood that suitable acid resistance is provided throughout and further, that insulation against loss of heat will improve the heat recovery and benefit the process.

In the normal operation of the sulphite process liquor at about 70 C. is introduced into the digester to cover the chips. The liquor is strong in calcium bisulphiteand free sulphurous acid. Steam is introduced and heating commences. The pressure rises gradfually over aperiod ofabout three hours to f about 70 pounds gauge pressure. The temperature increases at the same timeto about 120 C. during which period it is necessary to bleed the digester through line 21 to accommodate the condensed steam. Gas is also relieved, preferably through line 19. This, gas at'first contains considerable air from the chips and a quantity ofsulphurous acid released by the heat. The relief materials are valuable cooking agents (except the air) which are not contaminated, because no cooking has commenced. They pass through the separator and are permitted to flow directly into the accumulator via line 40. Considerable bleeding thus takes place prior to cooking, the separator perhaps emptying itself of liquid several times. Relief is continued after cooking has started-a period of about three and one-half to four hours after heating begins. When the digester attains a cooking temperature at about 140 C. the liquid bleeding decreases because the condensation is less, but the gas loss continues, and-gas and vapor are released continuously from the digester. Cooking continues under these conditions for a period of from three to seven hoursor more, according to particular mill practice.

During the first part of the bleeding and the release of gas, the coil line through accumulator 28 is closed at valve 38. The raw liquor in the accumulator is thus heated and en.- riched by admixture with the free acid gas of the relief and the relieved cooking liquor.

Pressure will gradually be! built up in the accumulator, to such an extent that the pressure reacts to retard relief. Relief cannot be maintained uniformly as-desired under these conditions. The discharge is therefore shifted so that it passes through the coil 37 and thereby gives upits heat. The shift is also made dependent upon the character of the discharge. When cooking has proceeded to such a stage that the impurities in the relief may contaminate the raw liquor, either because of their character or because of their amount, the coil is used to carry off the rethe materials with the liquor in the accumulator. The gas passes on through the coil to the cooler and thence to the reaction towers for complete utilization.

By operation as described the charge in the accumulator may be heated up to about 135 C. at a corresponding pressure. When the digester 10 has been emptied of its cook, it is filled with chi s from the supply 11, the to 12 is sealed, and the digester is vented, pre erabl through the separator and the by-pass 45. alve 35 is closed and valves 18 and33 opened, so that the pressure behind the preheated supply in the accumulator forces it into the digester. The chips cool the liquor to about 120 C. which temperature corres onds to the temperature of the normal cook beginning at 70 at the end of the first two hours. Thus, the accumulator permits reduction of the preheating time by two hours, prevents decreasing the strength of the liquor by the bleeding and dilution that.

is eliminated by cuttin off the two-hour pe- In case the accumulator charge is too small for one cooking operation, the additional liquor required is preferably supplied to the digester containing the chips prior to the admittance of the pressurecharge.

Several features of the process should be.

emphasized. All the relief gas is utiliz'ed l chemically in the preparation of raw liquor and little of the heat is wasted. No fumes are discharged. The hot cooking liquor is discharged under pressure into the digester, by gravity, by its own pressure, or by both, without the use of pumps. It has heretofore been difficult to charge such liquors when hot into a digester by means of pumps because of the corrosive action on the pumps and the packing. At the first part of the operations clean gas and liquor may be discharged into the accumulator to save the material and the heat content. This should not be continued throughout the cook, because t-he back pressure prevents proper relief, and because the relief material becomes contaminated with impurities removed from the wood. Admixture of relief with cooking liquor is made prior to the cook, may be made durin the first part of the cooking stage, and pre erably never during the last part of the cooking stage. The shift of valves-so that the material passes through the accumulator coil rather than into the accumulator, serves not only to conserve and to utilize the heat, but it also permits maintaining a uniform rate of relief. This permits of the proper control of the cooking operation. During the latter period of operation the liquor bled from the digester is much less in quantity, and the separator 24 serves to retain it and to separate it from the gas and vapor relief. The separator thus holds many of the impurities in the discharge for subsequent disposal.

The pulp from the shortened process is improved-in part due to the rapid penetration of the chips by the hotter and stronger liquor, and in part due to the quicker action of the stronger. liquor in the impregnated chips. Overcooking of the outer part of the chips is reduced, and the pulp is characterized by a freedom of the mass from knots and undercooked portions.

It is to be understood that there may be several digesters connected with one accumulator, and that a battery of digesters and of accumulators may be interconnected for selective operation. The example herein given is not to be considered as a limitation of the invention to the apparatus and the process specifically described anddisclosed, as it is contemplated that various departures and modifications may be made falling within the scope of the invention as expressed in the appended claims.

It is further to be understood that the process is not limited to the enriching of liquor for the sulphite cooking operationas described. Other liquors which are benefited by addition of material contained in the relief are contemplated, such for example as the enriching of any sodium sulphite liquor by the sulphurous acid relieved from the digester, or by the liquid bled from any process using a soda base. Preferably, however, the liquor to be preheated is that which is to be used in the next cooking operation. Accordingly, in the appended claims where the term liquor is employed it is 'to be given its broadest meaning.

We claim:

1. The method of treating cooking liquors for the sulphite process of making pulp which comprises relieving a sulphite digester during its operation, discharging the relief materials under pressure into a confined volume of liquor subsequently cutting off said discharge, continuing relief of the digester, and transferring heat from the relief material to said volume of liquor.

2. The method of treating liquor which comprises relieving a sulphite digester during its operation, discharging the relief material under pressure into a confined volume of liquor, subsequently cutting off said discharge when the back pressure interferes with the relief of the digester, and continuing relief of thedigester.

3. Themet-hod of treating liquor which comprises relieving a sulphite digester during its operation, discharging the relief material under pressure into a confined volume of liquor, subsequently cutting off saiddischarge when a predetermined injurious amount of impurities from the cooking digester are present in the relief materials, and continuing relief of the digester.

4. The method of recovering relief material from a sulphite digester which comprises relieving the digester, discharging the mate rial removed by relief prior to the start of cooking stages into a supply of liquor, and utilizing the heat in the material removed by relief during the cooking stage bytransfer methods.

5. The method of recovering relief material from a sulphite digester which comprises relieving the digester, discharging the material removed by relief prior to the cooking stage into a supply of sulphite cooking liquor, and utilizing the heat in the material relieved during the cooking stage by transfer methods to heat said sulphite cooklng liquor.

6. The method of recovering relief material from a sulphite digester which comprises relieving the digester, discharging the relief materia into a supply of raw sulphite cooking liquor, cutting off said suppl separating liquid from gas and vapor of the relief material, transferring the heat from the gas and vapor to'said liquor, and subsequently using the liquor for a sulphite cooking operation.

7. An apparatus of the character described, comprising a digester, an accumulator. means for discharging relief fluids from the digester into a supply of raw cooking liquor in the accumulator,a separator interposed in the last mentioned means for separating relief liquid from relief gas and vapors, a heat'exchanger in the accumulator and communicatin with said means, valves interposed in said means for cutting ofi the discharge of the relief fluidsvinto the accumulator or into said heat exchanger, and a valved conduit for 

